1. Field of the Invention
The present invention relates to a structure metallized with tungsten (W) which is formed on an aluminum nitride (AlN) substrate, and more particularly, relates to an improvement for increasing the junction strength of a W layer with respect to an AlN substrate.
2. Description of the Background Art
AlN is an electrical insulating material which has high heat conductivity, i.e., heat dissipation capacity. Therefore, a substrate of such AlN is advantageously applied to electrical equipment which requires heat dissipation.
In order to form a metallized structure on such an AlN substrate for providing a conductive path, a metal for example, tungsten is employed as the metallizing metal in consideration of heat resistance.
However, since AlN is inferior in wettability with tungsten, a tungsten layer formed on the AlN substrate is reduced in junction strength with respect to the AlN substrate. Further, when the AlN substrate is subjected to a heating step and a following cooling step, the tungsten layer is at least partially separated or the AlN substrate is partially damaged due to differences in the thermal expansion coefficient between AlN and tungsten. This further prompts the aforementioned reduction in junction strength.
In order to solve such problems, Japanese Patent Laying-Open No. 281089/1986 has proposed a technique of interposing a layer of a mixture of AlN and tungsten between an AlN substrate and a tungsten layer. Such an intermediate layer of the mixture is adapted to improve the junction strength between the AlN substrate and the tungsten layer.
However, it has been clarified through experiments made by the inventors that the junction strength with respect to the AlN substrate is gradually reduced particularly by repetition of heating and cooling steps in the aforementioned metallized structure.